Used plastics or plastic wastes usually contain high percentages of foreign substances, e.g., metal parts, paper remnants, glass, secondary plastics and the like. These foreign substances or impurities must, as a rule, be removed before the recycling of the plastics. This is frequently performed by the used plastics being at first plasticized by heating and by the plastic melt being subsequently filtered. So-called melt filters, by which the metallic or nonmetallic foreign substances or higher-melting plastics are separated, are used for this. However, the melt filters must be constantly cleaned to make continuous and trouble-free filtration possible.
A device of this type with a filter arranged in a filter space of a housing with a stripper for lifting off impurities retained by the filter and with a discharge means for removing the impurities lifted off from the filter by the stripper from the housing is known from DE 202 10 115 U1. The discharge means comprises in this prior-art device a feed screw, which is arranged within the housing in the immediate vicinity of the stripper and extends in parallel to the axis of rotation of the tubular filter. The feed screw is arranged such that the residues or impurities lifted off by the stripper from the outside of the filter are transferred directly to the feed screw and are removed by this from the housing. One drawback of this conveying means is that the impurities must be transported over the entire length of the feed screw and hence over a relatively long conveying path. In addition, there is a direct connection from the filter space in the housing via the feed screw in such a conveying means, so that, on the one hand, overflow of the material being conveyed into the feed screw intended for removing the impurities may occur.